Allow generation of a chromium outdirs when enable_extensions=false

extensions/browser/api/system_network/BUILD.gn

Index: extensions/browser/api/system_network/BUILD.gn
diff --git a/extensions/browser/api/system_network/BUILD.gn b/extensions/browser/api/system_network/BUILD.gn
index 7e67885e9b096a2cafccd042f543d296538c7acd..b7b1640783d86fc29c4961726808864a47ed590a 100644
--- a/extensions/browser/api/system_network/BUILD.gn
+++ b/extensions/browser/api/system_network/BUILD.gn
@@ -2,6 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//extensions/features/features.gni")
+
+assert(enable_extensions,
+       "Cannot depend on extensions because enable_extensions=false.")
+
 source_set("system_network") {
   sources = [
     "system_network_api.cc",